Two reasons:

1. I usually haven’t read the book involved, and so I don’t find endless debate to be anything much above prattling based on anything more than one tribe not liking a specific author.
2. When I have read the book, the nature of the opposite criticism is usually of such fervor that I can’t be bothered to give those opinions much thought. Why? I like to keep my blood pressure down.

I’m glad that some people enjoy these things; I do not.